LISBON, Jan 13 (Reuters) - Portugal"s ruling Socialists extended their lead in a new opinion poll ahead of a Jan. 30 snap election but remained short of a majority, while another survey released on Thursday showed the main opposition party narrowing the gap. The first survey, published late on Wednesday, carried out by pollsters Pitagorica for TV channels TVI and CNN Portugal, gave the current premier Antonio Costa"s PS party a 39.6% share of the vote, up from 37% last month. Although it was one of the best recent results obtained by PS it leaves them just short of a majority, which under the proportional representation system, equates to between 42% and 45% of the vote. The poll put the main opposition party, the Social Democrats (PSD), at 30%, down from 31.7% in December. The second survey, by Intercampus for newspapers Jornal de Negocios and Correio da Manha, showed PS remained favourites to win but only grabbing 29% of the vote, edging down from 29.4% last month. It gave PSD 24.1%, up from 22.2%. Intercampus said the number of undecided voters was around 18%. In October, Costa"s two former allies - the Communists and the Left Bloc - sided with right-wing parties to reject the minority government"s budget bill, triggering the snap election. If PS fails to win a full majority they will need the support of another party to pass legislation. The Left Bloc and the Communists saw support at 6.4% and 5% respectively in the Pitagorica poll, and 7% and 4.9% in the Intercampus survey. According to both polls, the Left Bloc remains the third largest party, closely followed by the rising far-right party Chega, polling between 4.9% and 5.7%. Pitagorica surveyed 600 people on Dec. 30 to Jan. 9, with a margin of error of 4%. The Intercapus poll was carried out on Jan. 4-10 and surveyed 615 people. It had a margin of error of around 4%.